35.3 A 30MHz Wideband 92.7dB SNR 99.6% Accuracy Bioimpedance Spectroscopy IC Using Time-to-Digital Demodulation with Co-Prime Delay Locked Sampling

The permittivity and conductivity of bioimpedance in tissues are governed by α, β, and y dispersions, with biological features detectable in the tens of MHz range. Wideband spectroscopy is required in many applications including body fluid volume assessment [1], cancer diagnosis [2], anatomical characterization [3], and plant content estimation [4]. However, state-of-the-art bioimpedance ICs are limited to frequencies up to 10MHz, which hampers biological research. For example, estimating starch content in cassavas requires wideband spectroscopy with frequencies up to 30MHz [4]. In a wideband system using analog I-Q demodulation, accuracy is limited by the large distortion from its multiplication process; a phase error of 4.32° at 10MHz was reported in [2]. In a wideband system using digital I-Q demodulation, a phase error of 0.15% at 10MHz was reported in [5] but at the expense of a large power consumption (12.2mW for the amplifier and ADC alone) and a mediocre SNR of 65dB. This paper presents a wideband (30MHz) bioimpedance spectroscopy IC achieving both high SNR (92.7dB) and high accuracy (99.6%) with low power consumption (3.18mW). The IC employs time-to-digital (T-D) demodulation and features: 1) a co-prime delay-locked (CDL) sampling approach achieving an equivalent sampling rate of 21.6GHz with a 240MHz sampling clock; 2) a delay-locked loop (DLL) architecture utilizing a compact phase comparator for high accuracy delay generation; 3) optimized wideband design for the current generator and readout front end to further reduce power consumption. The IC has 0.39% magnitude error and 0.57° phase error at 30MHz bioimpedance spectroscopy measurements.